When manufacturing chemical cellulose pulp from chopped chips, it is desired to expel air and moisture from the chips. It is at the same time desired to heat the chips to the desired process temperature, suitably to a level around 100° C., since the chips are finally to reach a temperature of approximately 130-160° C. during the cooking process. This requires large volumes of steam, since not only is the correct chip temperature to be achieved with the aid of the steam, not only is the bound air to be expelled by the steam, but also the bound chip moisture is to be heated.
In certain older conventional systems, atmospheric chip bins have been used in which the chips are pre-heated with steam in order to expel the air. Very large volumes of withdrawn air are obtained from these systems, which volumes are contaminated with turpentine, methanol and other explosive gases that have been expelled from the chips, the latter being denoted by the term “NCGs” (where “NCG” is an abbreviation of “non-condensable gas”). If steam is used that has been obtained from the release of pressure of black liquor, this steam contains also large quantities of sulphides, known as TRS gases (where “TRS” is an abbreviation of “total reduced sulphur”), which are very malodorous. These TRS gases contain, among other compounds, hydrogen sulphide (H2S), methyl mercaptan (CH3SH), dimethyl sulphide (CH3SCH3), dimethyl disulphide (CH3SSCH3), and other strongly malodorous gases. Hydrogen sulphide and methyl mercaptan, which principally come from the steaming of black liquor, have boiling points of −60° C. and +6° C., respectively, and it will thus be difficult to condense these compounds out from the gases.
Pure steam is often used for heating in the chip bin in order to minimise the release of TRS gases, and black liquor steam is used first in the subsequent steam-treatment step that follows the chip bin. Even if black liquor steam is used only in a subsequent steam-treatment step, it is still possible that these TRS gases leak up into the chip bin or are deliberately allowed to escape up into this chip bin during, for example, interruptions in operation.
Systems are revealed in U.S. Pat. No. 6,375,795 and in U.S. Pat. No. 6,284,095 in which it is attempted to disperse TRS gases from a pressure isolation device arranged between a chip bin and a steam-treatment vessel, where the TRS gases are withdrawn from the pressure isolation device and reintroduced at a position that lies downstream in the input sequence, at the outlet end of the steam-treatment vessel. The system has a chip bin arranged upstream, and a ventilation system is arranged at this bin in order to deal with weak gases. The system also provides possibilities for the dispersion of the TRS gases on certain occasions, either at a standpipe into the atmosphere, or to lead these TRS gases to the superior chip bin. Both of these alternatives involve the risk that TRS gases leak into the surroundings and create odour problems. The dispersal of pressurised TRS gases from the pressure isolation device, however, is combined with problems, since chips and fragments of chips can readily become stuck in the system, resulting is malodorous TRS gases being released up into the chip bin.
The prior art technology has identified the problem that it is desired to minimise leakage of harmful and toxic gases that arise during the steam pre-treatment with hot steam. It is normal to allow removal of weak gases from the chip bin to a destruction system, and to allow a further dispersal of gases from the steam pre-treatment vessel, the latter often being considered to be strong gases. It is attempted to maintain the concentration of the weak gases at well under 4% by volume, and the concentration of the strong gases at well over 40% by volume.
In the previously known chip bins in which steam is blown into the bed of chips, large volumes of weak gases are formed, and either pure steam or special systems that manage to deal with these weak gases are required. It is a property of weak gases that they very readily obtain a very explosive composition. As long as the concentration of NCGs lies lower than approximately 4% by volume or well over 40% by volume, there is no risk of explosion. For this reason, weak gas systems that maintain the concentration below under 4% by volume, typically below 1-2% by volume, or strong gas systems that maintain the concentration well over 40% by volume are used. It is thus ensured that the concentration in weak gas systems is held well below 4% by volume, and this entails the transport of large volumes of air: as soon as the volume of NCGs is set to increase, an equivalent increase in the fraction of air must be carried out in order to maintain the concentration below the critical limit.
If, for example, 1 kg/min of NCGs are steamed off in a chip bin, the air amount must lie around approximately 50 kg/min in order to maintain the concentration at approximately 2% by volume. If an increase in the NCGs to 2 or 3 kg/min takes place, as may occur in certain interruptions in the process, it is necessary temporarily to increase the amount of air to 100 or 150 kg/min. This results in the system being normally dimensioned such that it can deal with the normal flow, and that excess gases are vented directly into the atmosphere through the vent pipe when interruptions in operation occur.
Another solution to minimise the volumes of weak gases is to control the flow of chips through the chip bin such that a stable plug flow through the chip bin is obtained, and the supply of steam to the chip bin is in this case controlled such that only the chips in the lower part of the bin are heated. This technique is known as “cold-top” control and is applied in systems that are marketed by Kvaerner Pulping AB under the name DUALSTEAM™ bin.
A number of very expensive solutions have been developed in order to reduce the explosiveness and toxicity of the weak gases. Different systems are revealed in, for example, WO 96/32531 and in U.S. Pat. No. 6,176,971, in which cooking fluid withdrawn from the digester generates pure steam from ordinary water. The use of totally pure steam for the steam pre-treatment of the chips reduces the TRS content in the weak gases, since the steam used is totally free from any TRS content.
These systems, however, inevitably give rise to energy losses and additional expensive process equipment.
The principal aim of the invention is to obtain a chip bin or similar vessel for the steam pre-treatment of chips in which the risks of leakage of weak gases are minimised and that is not associated with the disadvantages of the prior art.
A second aim is to obtain a safe system with simple regulation in which it is ensured that the weak gases that are drawn from the chip bin always maintain a concentration of TRS gases (or of NCGs) that lies well below the level at which the mixture of gases becomes explosive.
The system uses a simple temperature regulation, in which, with increasing temperature of the weak gases, a gradually increasing amount of dilution air is added at the ventilation channel in which the weak gases are transferred to the destruction system or the DNCG system (where “DNCG” is an abbreviation for “diluted NCG”).
A further aim is to use a condensation arrangement in the weak gas system such that the gas volumes can be reduced early in the weak gas system, in which way an effective reduction in the volumes of weak gases can be achieved if large flows of steam are suddenly emitted from the top of the chip bin, and to avoid in this manner the customary venting to atmosphere. Current weak gas system are normally dimensioned such that they are able to deal with a nominally interruption-free flow of exhaust gases, and not to be able to deal with the increased volume of NCGs that may temporarily arise in the event of an interruption in operation. The volumes of gases obtained during such interruptions of operation are much larger than those that the weak gas system can manage, and the extra gas volume has, in general, been emitted to the surrounding air, through a dispersal standpipe of the roof of the mill, which has had as a consequence that the pulp mill has been compelled to emit malodorous gases.
A further aim is that the safety system is preferably used during what is known as “cold-top”-regulation of the heating of the chips, in which the chips are heated in such a manner that a temperature gradient is formed in the volume of chips, where the chips at the top of the chip bin maintain a temperature of approximately 40° C., and successively higher temperatures down towards the bottom of the chip bin are established with an advantageous temperature of approximately 90-110° C. established at the bottom of the chip bin. This system ensures that the volumes of gases that are expelled from the chips in the chip bin are very low, and the load on the weak gas system will be minimal during continuous routine operation. The system does, however, possess the property that NCGs tend to accumulate in a condensation layer in the chip bin, and in the event of steam break-through, when the chips reach a temperature of well over 40° C. at the top of the chip bin as a result of interruptions in the system, large amounts of NCGs are expelled from the bed of chips, which amounts must be dealt with by the weak gas system.